Hollow fiber type battery cells--as exemplified by high temperature, sodium/sulfur cells--have heretofore utilized disc-shaped tubesheets. See U.S. Pat. Nos. 3,476,062; 3,672,995; 3,697,480; 3,703,412; 3,749,603; 3,765,944; 3,791,868; 3,829,331; 3,917,490; 4,050,915; 4,219,613; 4,224,386; 4,296,052; 4,332,868 and 4,403,742--the disclosures of which are incorporated herein by reference, for all purposes which legally may be served thereby.
In the cells disclosed in the latter patents, the fibers terminate and open upon the "outer" or "upper" face of the tubesheet and depend as closed-ended lengths from the inner or lower face. The open ends communicate with an anode compartment and the closed-ended portions--together with intervening wraps of a current collecting/distributing aluminum foil--are immersed in the catholyte. The tubesheet periphery is sealingly engaged with at least the anolyte container and thus separates the anode and cathode materials; it is also electronically non-conductive. The fiber walls are "permeable" to cations of the anode material (molten sodium, for example), and the (conductive) anode material and the foil are connected by electrical lead posts to an external electrical circuit when the cell is in operation.
Particularly pertinent to the present invention are two patents in the preceding list, namely, U.S. Pat. Nos. 4,219,613 and 4,296,052. The '613 patent is directed to an assembly of a hollow fiber bundle and a helium-tight tubesheet of graded porosity (the latter resulting in better stress distribution on the fibers where they "enter" the tubesheet). The '052 patent concerns a two-step ("bake and broil") method of attaining the graded porosity during "firing" of the tubesheet, and constitutes the nearest known prior art firing method.
Difficulties have been encountered in attempting to scale up cells of the type disclosed in the above-listed patents. For example, as a consequence of the sizes of some of the particles from which the tubesheets employed must be formed and of the low glass transition temperatures of the solder glasses which have been found suitable for the methods of tubesheet fabrication used, the disc-shaped tubesheets employed in the prior art cells are too easily deformed, at elevated cell operation temperatures. Under the combined influences of gravity and the pressure differential resulting from transfer of material from the anode compartment to the cathode compartment during discharge, deformation occurs. The extent of deformation in a single discharge half-cycle is not large but the incremental deformation is not effectively reversed during recharging of the cell. Thus, deformation accumulates during prolonged charge/discharge cycling of the cell. This problem (which is not disclosed or suggested in the prior art) would be expected to become rapidly more severe as the diameter of the tubesheet is increased. Since as little as about a 10 mil (254 micron) deformation may cause fiber breakage, it will be appreciated that deformability is a real obstacle to scale up of cells in which the tubesheet is of the disc type.
Another, considerable, obstacle is the difficulty of fabricating larger, leak-free assemblies of hollow fiber bundles and disc-type tubesheets by the one-step method disclosed in the prior art. That is, when a "ladder" of parallel fiber lengths, wrap-spacing and fiber-spacing foil tapes and a cathode foil are rolled up together around a rotating, horizontal mandrel, the portions of the fibers which are to extend through the tubesheet extend substantially beyond the end of the mandrel. These protruding portions are unsupported and are deflected downwardly as the tubesheet material (a pasty slurry of glass particles in a volatilizeable liquid medium) is deposited on (and between) them. As the developing roll rotates, the sagging fiber portions are lifted, bowed even more and subjected to a twisting action. The resultant flexing makes proper placement of more slurry awkward at best and is not very helpful to formation of a body in which the slurry is uniformly distributed around and between the fibers; it is difficult to prepare tubesheet/fiber assemblies by this method which do not require some type of post-firing treatment to render them leak-free. This is particularly so for larger diameter tubesheets of the prior art type.
It is possible to make useable disc-type tubesheets by other than the "bake and broil" method. That is, the advantages of a graded porosity in the tubesheet may be dispensed with for the sake of the lower number of leak paths which results when the tubesheet is more uniformly densified. However, this requires extremely critical, close control of the firing time and temperature, in order to ensure adequate melting of the tubesheet material and bonding to the fibers, without closing off an excessive proportion of the fibers at the same time.
Thus, it is apparent that a tubesheet configuration which would avoid the foregoing problems is highly to be desired.
Another problem with disc-type tubesheets--particularly the more highly densified versions thereof--is that substantial contraction occurs as the liquid slurry medium is removed and densification occurs. The greatest displacement, by circumferential and radial shrinkage, occurs in the peripheral portion of the tubesheet, resulting in "mud-cracking". Thus, as the tubesheet diameter is increased, the cracking becomes so extensive as to render the disc non-functional for its intended purpose.
It will be seen that the most readily apparent solution to the deformation problem, thickening the tubesheet, would aggravate, at least, the difficulty of forming the "green" tubesheet/fiber (etc.) assembly. However, if the amount of the tubesheet material could be reduced, i.e., if the spacing between the fibers within the tubesheet could be considerably reduced, an elongated tubesheet of correspondingly reduced diameter would result. The tubesheet would be highly resistant to deformation. But another problem is posed. The latter modification would require holding the fiber ends together during introduction of the slurry or "squeezing out" some of the slurry; neither expedient worked when tried. It would also necessitate spacing the tubesheet further out along the mandrel axis from the rest of the assembly, to avoid excessively sharp bending of the outermost fibers, particularly upon scale up to larger diameter assemblies. This would increase the sagging and flexing during rolling, due to the greater leverage and even though less slurry was applied.
In addition, the spherical particles included with the ground glass in the tubesheet slurry (to ensure extrudeability of the pasty slurry and efficient, more uniform particle packing in the green tubesheet) would have to be removed in order to attain close packing of the fiber ends. This in turn would require using a more dilute slurry (to retain extrudeability) and would have the consequence of even greater shrinkage during drying and firing of the tubesheet. It would also make initial retention of the slurry on the rotating fiber "brush" much more difficult.
Yet another apparent difficulty with going to an elongate or "plug" tubesheet configuration is that the butt-type seals employed in the prior art for joining the cathode (and anode) cup(s) to the periphery of the disc-type tubesheet would not appear to be very practical for relatively small diameter tubesheets. This would necessitate using an overlapping, concentric ("sleeve") type of seal and such seals are notoriously more difficult to form as strain-free bodies.
It will be recognized that a plug tubesheet--if somehow attainable--would have a substantial safety benefit. That is, breakage of a disc-type tubesheet in an active cell can result in immediate contact between relatively large amounts of anode and cathode materials and an ensuing, highly exothermic, chemical reaction; temperatures so high as to initiate an extremely vigorous reaction between the sulfur in the catholyte (in a sodium/sulfur cell, for example) and the aluminum foil may result. In contrast, breakage of a plug tubesheet (at least in a "two-compartment" cell) would result in exposure of only very limited amounts of the anode and cathode materials to contact with each other.
Thus, despite the uncertainties posed by the several demonstrated and contemplated difficulties of making plug tubesheets, a workable method of fabricating them was still sought.